Mosfet with reduced threshold voltage and on resistance and process for its manufacture

ABSTRACT

A vertical conduction MOSFET having a reduced on resistance R DSON  as well as reduced threshold voltage V th , and an improved resistance to punchthrough and walkout has an extremely shallow source diffusion, of less than 0.3 microns in depth and an extremely shallow channel diffusion, of less than about 3 microns in depth. In a P channel version, phosphorus is implanted into the bottom of a contact trench and into the channel region with an implant energy of 400 keV for a singly charged phosphorus ion or 200 keV for a doubly charged ion, thereby to prevent walkout of the threshold voltage.

FIELD OF THE INVENTION

[0001] This invention relates to power MOSFETs and more particularly to a novel process and resulting product which allows a reduction in threshold voltage while retaining a low on-resistance.

BACKGROUND OF THE INVENTION

[0002] MOSFETs and processes for their manufacture are well known. A typical device and process for its manufacture is disclosed in copending application Ser. No. 09/436,302, filed Nov. 8, 1999 entitled LOW VOLTAGE MOSFET AND PROCESS FOR ITS MANUFACTURE AND CIRCUIT APPLICATION (IR-1531) which is incorporated herein by reference.

[0003] One limitation encountered in making low voltage (under 50 volts), low on resistance (R_(DSON)) MOSFETs is the Rdson test condition at a low V_(gs). To provide a low V_(gs) gate drive condition the channel component of Rdson becomes increasingly larger, eventually becoming larger than all of the other components combined. There are several known ways of reducing this channel component of on resistance under these circumstances but one of the most direct is to lower the threshold voltage V_(th), ensuring that the device is fully on at the low V_(gs) condition. In many cases the gate oxide thickness can be reduced to lower the threshold voltage. However, when this thickness is fixed due to V_(gs) max concerns, other means must be used.

[0004] The threshold voltage, V_(th), can also be reduced if the channel dose is lowered. In addition, the channel junction must be shallow so that the channel length can be kept short. To maintain the shallow channel junction, the overall thermal budget, that is, the cumulative diffusion drive cycles, must be low. This introduces constraints on the shape of the source region so that its shape is more “oblong” (or elongated in depth) in the vertical direction relative to the substrate surface than under other conditions. This creates a region in the device where the corner of the source region is very near the “corner” (in cross-section) of the channel junction.

[0005]FIG. 1 is an idealized sketch of a vertical conduction MOSFET cell after the formation of the source and channel diffusions and a trench for a contact. The structure shown is for a P channel device, although all conductivity types can be reversed for an N channel device. In FIG. 1, a P type epitaxially deposited substrate (and drain region) 10 receives, in a conventional DMOS process, an N type channel implant and diffusion 11 and a P type source implant and diffusion 12. The window in gate structure 13 is the implant widow for both regions 11 and 12. Note that the source junction 12 is very close to channel junction 11 at the corner of the device cell, shown as distance “X”. Thus, because, thermal treatment after the formation of channel junction 11 must be kept to a minimum, there is relatively little lateral diffusion of the source junction 12 under gate 13. The shape of source junction 13 is therefore deeper with less lateral spreading than usually found, i.e. more “oblong” than a typical diffused junction. Consequently, dimension “X” is reduced so that, when the diode junction 11 is reverse biased, punchthrough occurs at low voltage.

[0006] The punchthrough condition introduces unnecessary leakage from the drain to the source. Punchthrough is a phenomena associated with the merging of the depletion regions of the source and drain junctions and occurs when the channel length is small, so that, as the depletion region forms in the reverse biased body diode, it extends to the source region where majority carriers are forced into the body diode depletion region, creating premature leakage from drain to source, before reaching the avalanche condition. Further, V_(th) was unstable with time, particularly with P channel devices, (similar to the condition known as walkout). The condition of walkout manifests itself with a gradually increasing voltage as a function of time, for the same applied current. The scale could be a few milliseconds to 100's of milliseconds (in the worst condition) and is very undesirable.

[0007] Referring to FIG. 1, it will be seen that a trench 20 is formed. A heavily doped, shallow N⁺ base 21 is implanted in the bottom of the trench to serve as a good contact region for the body diode. The trench 20 is especially useful in P channel devices. Body 21 has been made in the past by a phosphorus implant employing normally used implant energies.

[0008] Thus, the resulting device in the prior art had a tradeoff of increased on resistance for low V_(gs); and further, P channel devices exhibited walkout. More particularly, in the prior art, it was known that V_(gs) could be reduced by lower channel concentration, at the cost of increased R_(DSON); or by reducing gate oxide thickness, at the cost of reduced gate ruggedness. Also, it was known that R_(DSON) could be reduced by using a shorter channel and using a deeper source, but that increased the likelihood of punchthrough.

[0009] It would be very desirable to provide a process and structure which resolves these tradeoffs in which the V_(gs), and R_(DSON) can be both reduced without creating a punchthrough condition or, in P channel devices, a walkout condition.

BRIEF SUMMARY OF THE INVENTION

[0010] In accordance with the present invention, both R_(DSON) and V_(th) are simultaneously reduced by the control of the positions and shapes of the source and channel regions and by the proper control of the spreading of the reversed biased depletion region.

[0011] This novel process and design revolves around three concepts:

[0012] 1) Controlling the relative position and shape of the source and channel regions. This is achieved by making the source implant as shallow as possible to limit punchthrough. Thus, the source implant energy is constrained to be as low as possible, right to the verge of beam blowup. This implant and its associated diffusion is done ideally within the framework of a very shallow channel junction. By way of example, the source and channel implants are preferably less than 0.3 μ and 3.0 μ, respectively.

[0013] 2) Controlling the reverse biased depletion region spreading by the amount and distribution of charge buried deep within the channel junction. For a p channel device, this is done by using a very high energy implant, in this case a 200 kV implant of doubly charged phosphorus ions P⁺⁺ (resulting in an effective 400 keV implant). This implant also helps to limit the occurrence of punchthrough. This is accomplished by a heavy base implant that is placed as deep as possible by implanting doubly ionized phosphorus at very high energies. This charge, when placed strategically, acts as charge balance for the reverse biased diode, limiting depletion spreading into the channel side (as opposed to the epi side) of the junction. Similar concepts apply to the N channel device.

[0014] 3) In addition, the extra depth of the 400 keV implant helps to make the V_(th) measurement stable with time. It is thought that when the heavy dose of the P⁺⁺ implant is deeper due to the high energy of the implant, that the increased charge makes it more difficult for a walkout condition to occur. Typically, walkout occurs in more lightly doped regions (e.g. it is often a problem in high voltage devices).

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows the structure of a single cell of a MOSFET of the prior art.

[0016]FIG. 2 shows a small area of silicon at a first stage of the manufacture of a cell in accordance with the invention and the formation of the channel junction.

[0017]FIG. 3 shows the section of FIG. 2 after the formation of the source region.

[0018]FIG. 4 shows the section of FIG. 3 after the implant of the heavy base implant.

[0019]FIG. 5 shows plural cells of the type shown in FIG. 4 in a MOSFET, along with its source, drain and gate contact electrodes.

DETAILED DESCRIPTION OF THE DRAWINGS

[0020] Referring to FIG. 2, there is shown a single cell of a vertical conduction MOSFET (which may have a stripe, or closed polygonal topology) in which, for a P channel device, a P⁻ epitaxial layer 30 of silicon has a passivation layer 31 thereon which has a window 32 cut therein. A channel region 33 (or junction 33) is then formed by the implant of phosphorus at 120 keV and a dose of 1.2E14 atoms/cm². This implant is then diffused for 30 minutes at 1175° C., forming the junction 33 to a depth which is less than about 3 microns.

[0021] Next, as shown in FIG. 3, a source region 34 (or junction 34) is implanted through window 32 at the lowest possible implant energy so that it is as shallow as possible. By way of example, source region 34 is formed by a boron implant at less than about 20 keV and 2E15 ions/cm², with source junction 34 having a depth of less than 0.3 microns.

[0022] A third implant is then carried out to form the N⁺⁺ contact implant 35 shown in FIG. 4. The contact trench may be first formed. The implant of phosphorus may be carried out at an energy greater than 350 keV (for a single charge phosphorus ion) and a dose greater than 1E15 ions/cm². The energy of the implant is more critical than the dose.

[0023] All implants are activated with activation temperatures of 1175° C. for about 30 minutes.

[0024]FIG. 5 shows a portion of a vertical conduction MOSFET employing the cells of FIG. 4. Note that the epi layer 30 is grown atop a P⁺ substrate 50. FIG. 5 further shows the gate structure as consisting of a gate oxide lattice 51 which extends across each of the channel regions, and a conductive polysilicon gate layer 52 atop lattice 51. The polysilicon gate 52 is sealed and insulated by an LTO (low temperature oxide) layer 53. A source contact 54 then overlies the top surface and contacts the source and channel regions of each of the cells. A drain contact 55 is disposed on the bottom of substrate 50.

[0025] The novel MOSFET so produced has both a low threshold voltage (below 2 volts) and a low R_(DSON) (below 5 mΩ) at low V_(gs) (which may be as low as 2.5 volts). This expanded “window” of values was previously highly constrained by I_(DSS), due to punchthrough on the one hand and low V_(th) on the other hand.

[0026] As a further aspect of the invention, it has been found that V_(th) walkout is eliminated for P channel MOSFETs. Further, for both P and N channel devices, the V_(th) is 50% lower than that previously attainable, thus dramatically improve gate robustness, gate rating and, at the same time, provide a reduced gate charge.

[0027] Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein. 

What is claimed is:
 1. In a vertical conduction power MOSFET comprising a die of monocrystaline silicon having an upper and lower surface and having a first conductivity type; a relatively thin layer of epitaxially grown silicon of said first conductivity type atop said top surface; a plurality of spaced channel regions of a second conductivity type diffused into the top surface of said epitaxially grown silicon layer; a respective source region of smaller area than said channel regions of said first conductivity type diffused into each of said channel regions and defining lateral invertible channels in the space between the peripheries of said channels and their respective sources; a MOSgate structure overlying each of said invertible channels; a source electrode overlying the top of said die and connected to each of said channel and source regions, and insulated from said MOSgate structure; and a drain electrode coupled to said epitaxially grown silicon layer; the improvement which comprises said channel diffusions having a depth less than 3 microns, and said source diffusion having a depth less than 0.3 microns.
 2. The MOSFET of claim 1, wherein said first and second conductivity types are N and P respectively.
 3. The MOSFET of claim 1, wherein said invertible channels have a length of less than about 1 microns, whereby the distance between respective pairs of said source and channel regions at their corner points of maximum curvature is about 2.5 microns.
 4. The MOSFET of claim 1, which further includes a rectangular trench extending through the center of each of said source regions and into its respective channel region; and a high concentration contact diffusion of said first conductivity type disposed in the bottom of said trench; said source contact filling said trench and contacting said high concentration diffusion.
 5. The MOSFET of claim 2, which further includes a rectangular trench extending through the center of each of said source regions and into its respective channel region; and a high concentration contact diffusion of said first conductivity type disposed in the bottom of said trench; said source contact filling said trench and contacting said high concentration diffusion.
 6. The MOSFET of claim 3, which further includes a rectangular trench extending through the center of each of said source regions and into its respective channel region; and a high concentration contact diffusion of said first conductivity type disposed in the bottom of said trench; said source contact filling said trench and contacting said high concentration diffusion.
 7. The MOSFET of claim 5, wherein said first concentration type is N and wherein said high concentration contact diffusion is a phosphorus diffusion formed with an effective implant energy of greater than about 350 keV for a singly charged phosphorous ion. 